Abstract The cellular decision to grow and divide relies on sensing mechanisms for growth factors and nutrients. Multiple cellular sensing pathways converge at the central signaling node of the mechanistic target of rapamycin complex 1 (mTORC1), a master regulator of metabolism. While amino acid and glucose deprivation are known to inactivate mTORC1 through well-defined mechanisms, it remains unknown whether sensing mechanisms exist for vitamins and cofactors, which are equally important for central carbon metabolism. Here, abundance of the essential nutrient Vitamin B5 (VB5) was shown to influence cell cycle progression and mTORC1 activation. VB5 is the precursor for Coenzyme A (CoA), an acyl chain carrier necessary for central carbon metabolism. However, response to perturbation of VB5/CoA metabolism is not due to cellular energy depletion, suggesting that the phenotype does not simply reflect decreased carbon source oxidation. While non-transformed cells undergo stable cell cycle arrest under selective VB5 depletion, cell lines lacking tumor suppressors involved in regulation of the cell cycle and mTORC1 instead undergo cell death. Therefore, cell cycle arrest and mTORC1 inhibition appear to be necessary for survival of perturbed VB5/CoA metabolism. If VB5 is sensed through a downstream metabolite, then patients with cancer cells lacking specific tumor suppressors, or tuberous sclerosis complex patients in which mTORC1 is dysregulated, could be potentially targeted with inhibitors of CoA synthesis. The proposed aims are designed to 1) identify the cell cycle regulatory machinery necessary for survival in VB5 depletion and 2) determine the molecular mechanism of VB5-mediated mTORC1 regulation by investigating the roles of mTORC1-regulating protein complexes and metabolites. Both aims will characterize sensitivity to inhibition of CoA synthesis in cells lacking the tumor suppressors involved in each pathway. These research questions will be addressed by genetic ablation of key cell cycle and mTORC1 regulators and perturbation of VB5/CoA metabolism through selective VB5 depletion and pharmacological inhibition of CoA synthesis. Biochemical and mass spectrometry techniques will be used to investigate the influence of VB5/CoA abundance on signaling proteins and metabolites. The proposed work will shed light on a novel vitamin-sensing mechanism and define its regulation of critical cellular growth and proliferative pathways, with the potential to reveal targetable cancer cell metabolic vulnerabilities.